1. Field of the Invention
This invention relates to photodetectors and more particularly to an array of photodetectors each having high dynamic range and low optical and electrical cross-talk between detectors.
2. Description of the Prior Art
In the prior art, arrays of photodiodes have beeen fabricated by diffusing an array of P+ regions into an N-type substrate. When the photodiode array receives radiant energy in the infrared region such as 7,000 angstroms, the cross-talk is significant. For example, an array of photodiodes spaced apart on a center to center spacing of 12 microns and having a junction and depletion depth below the substrate surface of 5 microns would have 15 decibels cross-talk to a photodiode located 36 microns away from a photodiode illuminated with radiation having a wavelength of 8500 angstroms.
It is known in the art to improve the isolation of an array of photodiodes in a substrate by fabricating P+ regions in an N-type epitaxial or diffused layer of a predetermined depth on a P-type material substrate. Cross-talk isolation between photodiodes is improved by action of the PN junction formed between the N-type layer and the P-type substrate by collecting carriers generated deep in the silicon substrate thus preventing the carriers from diffusing to neighboring or adjacent photodiodes. The cross-talk isolation between one photodiode and another photodiode spaced 36 microns away would be about 30 decibels. One disadvantage of the PN junction between the N layer and the P-type substrate is the lower quantum efficiency resulting from loss of carriers generated in the PN junction or below.
A disadvantage of the above-described photodiode array structures is that the photo current resulting from impinging radiant energy at a photodiode may be of sufficient magnitude to completely discharge the reverse biased floating diode and cause it to become forward biased. A forward biased photodiode would inject minority carriers into the N layer which would diffuse to neighboring photodiodes causing their discharge resulting in blooming.
An array of photodiodes formed on an N-type substrate is described in U.S. Pat. No. 4,160,985 which issued on July 10, 1979 to T. I. Kamins and G. T. Fong. An array of P regions are diffused in an N-type layer which has a higher conductivity N-type layer below on an N-type substrate. The high conductivity N+ type layer reduces cross-talk between photodiodes by accelerating photogenerated charge carries toward the surface or P+ regions so that the carrier diffusion to distant photo sensing elements is minimized.
The use of narrow grooves or slots etched into the substrate material containing an array of photodetectors to provide electrical and optical isolation between the photodetectors is described in U.S. Pat. No. 4,106,046 which issued on Aug. 8, 1978 to H. C. Nathanson and M. M. Sopria and assigned to the assignee herein. In U.S. Pat. No. 4,106,046, elongated detectors formed in a silicon substrate were separated from one another by an isotropic etch on (110) surface oriented silicon wherein the grooves were aligned with the [111] planes to provide a depth to width ratio on the order of 400 to 1 when aligned within 1/4 degree with respect to the [111] plane. One example of an etchant suitable for etching the silicon was described as 44% KOH:H.sub.2 O at a temperature of 85.degree. C. and below. The grooves provide electrical isolation by physically separating adjacent detectors and provide optical isolation by the side walls of the grooves which reflect radiant energy from within a detector to prevent radiant energy from passing into an adjacent detector.
It is therefore desirable to provide an array of photodetectors, each detector having high dynamic range with low electrical and optical cross-talk between photodetectors.
It is further desirable to provide an array of photodiodes wherein each may operate in the reverse biased integrating mode for low level signals and in the forward biased photovoltaic mode for high level signals while preventing electrical and optical cross-talk and blooming.
It is further desirable to provide an array of photodiodes having electrical and optical isolation provided by a PN junction below the photodiode which may remove carriers passing through the forward biased diode by bipolar transistor action.
It is desirable to provide photodiodes which are electrically and optically isolated from one another by open or oxide filled grooves to prevent lateral diffusion of photogenerated carriers.